1. Field of the Invention
The present invention relates to a sensor control device for controlling the operation of a sensor element made of solid polymer electrolyte (SPE) capable of measuring a gas concentration of a specific component contained in a target gas in a wide range.
2. Description of the Related Art
Such a type of a sensor control device for controlling the operation of a sensor element made of solid polymer electrolyte (SPE) is concretely realized or used as an A/F ratio (A/F ratio) detection device capable of detecting an oxygen concentration in the exhaust gas of (or combustion gas) emitted from an internal combustion engine mounted on a motor vehicle, for example. Such an A/F ratio is the mass ratio of air to fuel present during combustion. The sensor control device transfers a detection result to an A/F ratio control system composed of an engine ECU (electric control unit). The ECU performs a stoichiometry combustion control of carrying out the feedback of the A/F ratio around a stoichiometry value (as a theoretical A/F ratio) or performs a lean combustion control of carrying out the feedback of the A/F ratio in a predetermined lean area.
More recently, there are demands such as automobile emission control and on-board diagnostics (or OBD), for example. The automobile emission control covers all the technologies that are employed to reduce the air pollution-causing emissions produced by automobiles. The OBD is a computer-based system built into vehicles and trucks. An OBD system is designed to monitor the performance of some of an engine's major components for controlling emissions. Those recent demands need to increase the performance of controlling the stoichiometry combustion and also to expand the A/F ratio detecting range toward the atmosphere condition in addition to the lean area. For example, there is a necessity to detect deterioration of sensors, such as a clogged state when a fuel supply is halted in usual operation, as OBD. It further becomes important to enhance the fuel consumption as well as improvement of exhaust gas emission control. It is therefore very important to perform the feedback control of a fuel rich condition in a high load operation of an engine.
The inventors according to the present invention have proposed improved techniques. For example, Japanese patent laid open publication No. JP 2004-205488 has disclosed a sensor control device equipped with a plurality of amplifiers having different amplification factors. In the sensor control device, A/F ratio detection signals are generated according to the outputs from the plural amplifiers. The above technique intends to detect the A/F ratio in a wide A/F ratio detection range and to increase the detection accuracy for the A/F ratio in a desired air/fuel detection range. The related art technique described above can detect an A/F ratio with high accuracy in the desired air/fuel detection range.
However, the related art techniques contain disadvantage, namely, need to incorporate the plurality of amplification circuits (composed of operational amplifiers) and therefore need to have a large circuit size and plural terminals through which signals are input and output. Thus, the related art techniques are still necessary to improve and eliminate the above disadvantage.
There is another related-art technique capable of setting a stoichiometry narrow range near a stoichiometry value and a lean narrow range in a fuel lean area in addition to switching the wide range and a narrow range as the A/F ratio detection range. For example, Japanese patent laid open publication No. JP 2006-275628 has disclosed a sensor control device, in which a differential amplification circuit inputs an element current signal flowing through a sensor element and then amplifies the input one by a specific amplification factor. In the sensor control device disclosed in JP 2006-275628, an offset voltage is supplied to one input terminal of the differential amplifier and the stoichiometry narrow range near a stoichiometry value and the lean narrow area in a lean value as the A/F ratio detection range by switching the offset-voltage.
FIG. 14 is a diagram showing a part of a configuration of a related-art sensor control circuit disclosed in JP 2006-275628 described above. FIG. 14 shows a circuit diagram which corresponds to FIG. 2 disclosed in JP 2006-275628. In particular, FIG. 14 omits a configuration capable of changing the signal amplification factor for brevity.
In FIG. 14, reference number 101 designates a shunt resistance as a current detection resistance through which a current flowing through an element is detected. Both ends of the shunt resistance 101 are connected to buffers 102 and 103, respectively. One terminal of a resistance 112 is connected to the buffer 102. The other terminal of the resistance 112 is connected to an inverting input terminal (as a negative (−) input terminal) of an operational amplifier 111 which forms a differential amplification circuit 110. A resistance 113 is connected between the negative (−) input terminal and an output terminal of the differential amplifier 111. One terminal of the resistance 114 is connected to the buffer 103. The other terminal of the resistance 114 is connected to a non-inverting input terminal (as a positive (+) terminal) of the operational amplifier 111. A switch 116 is connected to the positive (+) input terminal of the operational amplifier 111 through a resistance 115. The switch 116 selecting one of the contacts connected to a node of a voltage V11 determined by voltage dividing resistances 116 and 117 and to a node of a voltage V12 (not equal to the voltage V11) determined by voltage dividing resistances 119 and 120. The voltage potential of the power source is divided by the four resistances 117 to 120.
In the sensor control circuit having the above configuration shown in FIG. 14, the switch 116 can switch the offset voltage for the differential amplification circuit 110 to the stoichiometry narrow range near the stoichiometry value and the lean narrow area in the lean area.
However, in the sensor control device disclosed in JP 2006-275628, the resistance value at the non-inverting input terminal (as the positive (+) input terminal) of the operational amplifier 111 is changed by the switching operation of the switch 116, and this results in the fluctuation of the amplification factor and the like of the operational amplifier 111. That is, when it is considered that the values of the resistances 112 and 113 connected to the inverting input terminal (as the negative (−) terminal) of the operational amplifier 111 are designated by R11 and R12, and the values of the resistances 114 and 115 connected to the non-inverting input terminal (as the positive (+) terminal) of the operational amplifier 111 are designated by R13 and R14, the resistance value determined by the dividing voltage resistances 117 and 118 are designated by R15, and the resistance value determined by the dividing voltage resistances 119 and 120 is designated by R16. In this case, the differential amplification circuit 110 can perform a signal amplification in a desired amplification factor (=R12/R11) unless the following condition is satisfied:
(1) R11=R13; and
(2) R12=R14+R15, or (3) R12=R14+R16.
However, when R15 is not equal to R16 (R15≠R16), one of (2) and (3) cannot be satisfied. This phenomenon has a drawback to vary the signal amplification factor and gas concentration detection range by the operation of the switch 16, contrary to the expectation.